Why Don’t Birds’ Legs Freeze

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How
a bird reacts to the cold depends on what kind of a bird it is and there are a variety
of methods they use to keep their legs warm, from the mundane to the much more fascinating. For starters, many birds will simply pull
their legs and feet close to their centre of mass one at a time, keeping them warm with
their feathers and body heat and off the cold ground. Likewise, some birds crouch down and cover
both feet with their plumage. In particularly cold weather, the bird can
also “fluff up” its feathers, trapping air between the feathery layers to keep extra
warm from head to toe, so to speak. If there are other birds around, they may
also huddle together when doing this. Another slightly mundane, but also very helpful,
method of keeping themselves, and in turn their feet, warm is body fat. Towards this end, some birds will gorge themselves
before winter, not just for the extra energy store, but to build up a layer of fat to help
shield themselves from the weather. Further, as a last resort, most birds can
shiver their flight muscles to generate body heat (not unlike how the cold-blooded honey
bee keeps the hive warm in winter), though this comes with the problem of physically
tiring the bird and perhaps limiting its ability to fly. Concerning the feet and legs more directly,
many bird legs are covered with a rough, scaly skin that limits heat loss, not to mention
the legs and feet often have an incredibly small surface area compared to the rest of
their bodies, severely limiting the amount of skin actually exposed to the cold. So what about something like a duck with positively
huge, flat feet that are not only exposed to the elements, but may be submerged in ice
cold water? In these cases, the birds have a nifty trick
up their non-existent sleeves. Using a counter current exchange system, some
birds can submerge their feet in ice cold water for hours at a time without real consequence. In a nutshell, counter current exchange simply
means that the bird’s veins and arteries are aligned in such a way as to facilitate
heat exchange between warm blood being pumped towards the feet and the cool blood being
pumped away from them. This means that recently cooled blood from
the bird’s feet is warmed before it reaches the bird’s core while at the same time blood
from the bird’s core is cooled significantly before it reaches the feet. This makes sure they aren’t dually heating
the blood returning to the core and the water around their legs and feet with their core
temperature blood (often 104 degrees Fahrenheit, 40 degrees Celsius, or more depending on the
bird). Instead, the lace-like formation of the birds’
capillaries ensures that a lower, but sustainable temperature is maintained in the feet (sometimes
only just above freezing, as is the case with Emperor penguins or ducks swimming in a nearly
frozen body of water), allowing it to endure freezing temperatures around their exposed
feet, while minimizing heat loss. Further, birds with this evolutionary trait
can actually control the flow of blood to their extremities using a series of muscles
located at the top of their legs. In extreme circumstances, some can even cut
off the flow of blood completely, though only for a short time without damage. They can also go the other way with it when
overheated to use their exposed feet and ability to control blood flow to cool themselves down
when hot. However, this system is far from perfect and
it’s noted that, for instance, if a duck is left in a warm environment before being
put directly into a freezing one, its feet can sometimes struggle to adapt and may result
in the poor little guy getting frostbite. Yes, despite all the methods of keeping their
feet warm listed above, ducks and other birds can get frostbite, and sometimes die from
the cold during particularly harsh winters. Bird poop is white due to their kidney’s
extracting nitrogenous wastes from their bloodstream and subsequently excreting it in the form
of uric acid, which has a very low solubility in water and emerges as a white paste-like
substance.

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